WASHINGTON — The Trump administration said on Friday that it was suspending one of the last major nuclear arms control treaties with Russia, following five years of heated conversations over accusations by the United States that Moscow is violating the Reagan-era agreement.

It also comes as the United States has begun building its first long-range nuclear weapons since 1991, a move that other nations are citing to justify their own nuclear modernization efforts.

Taken together, the two moves appear to signal the end of more than a half-century of traditional nuclear arms control, in which the key agreements were negotiated in Washington and Moscow.

It is unclear whether President Trump plans to replace the I.N.F. or to renew another major treaty, called New Start, which drove American and Russian nuclear arsenals to their lowest levels in nearly 60 years. That accord expires in 2021, just weeks after the next presidential inauguration.

Secretary of State Mike Pompeo announced the decision to suspend the accord, declaring that “countries must be held accountable when they break the rules.”

“We can no longer be restricted by the treaty while Russia shamelessly violates it,” Mr. Pompeo said, adding that the United States would terminate the accord in six months unless Russia destroyed its growing arsenal of intermediate-range missiles and launchers.

Mr. Trump said later that “I hope we’re able to get everybody in a big, beautiful room and do a new treaty that would be much better.” He did not define what he meant by “everybody.”

The Russian government counteraccused the Trump administration of looking for any excuse to end the Cold War-era agreement. Dmitri S. Peskov, the spokesman for President Vladimir V. Putin, said the United States failed to negotiate in good faith.

“On the whole, the reluctance of the Americans to listen to reason and to hold any kind of substantive talks with us shows that Washington decided to crush the treaty a long time ago,” Mr. Peskov told reporters.

In a series of public and private comments, the president and Trump administration officials have made clear they are seeking a new strategy that would revive the treaty — but only if all countries that now field such weapons are willing to curb or eliminate them. The current agreement applies only to Russia and the United States.

That would be an enormously ambitious task. It would require China, India, Pakistan, Iran and North Korea to sign on to the same agreement. That is much tougher than negotiating a bilateral treaty between Russia and the United States, which still possess, by far, the world’s largest nuclear arsenals.

While the administration has not yet formally announced any such effort, a range of officials pointed to language used by Mr. Trump at the Pentagon last month to embrace a new missile defense strategy. “For too long, we have been held back by self-imposed limits while foreign competitors grow and they advance more than we have over the years,” he said.

Some experts are skeptical.

“Nobody in the administration has laid out what the action-reaction cycle looks like as the United States makes all these moves — building new warheads, withdrawing from treaties, pursuing new missiles,” said Jon Wolfsthal, a nuclear expert on the National Security Council during the Obama administration.

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President Xi Jinping of China and President Vladimir V. Putin of Russia in September in Vladivostok, Russia.CreditPool photo by Alexander Ryumin

“The enemy gets a vote,” he added. “The idea that we’re going to do these things, and they’re going to stand still, is nonsensical. They’re going to respond for both political and military reasons.”

One thing is clear: If a new arms race begins, it will be expensive. While the makeover of America’s aging nuclear arsenal and laboratories began during the Obama administration, the ambitions to remake the United States’ nuclear capabilities have accelerated under Mr. Trump.

For the next 10 years, the Congressional Budget Office said in January, the cost of nuclear upgrades has increased to $494 billion, or 87 times the amount Mr. Trump is seeking for his border wall. Over the next 30 years, the estimate is $1.2 trillion.

The move on Friday to suspend and likely end the I.N.F. treaty, once considered the gold standard of arms control agreements, was telegraphed months ago.

The United States began accusing Russia of violating the treaty in 2014, when it revealed that Moscow was developing a new missile that would violate the range limits. The Russian government maintains that missile battalions it has deployed near European borders would not fly far enough to violate the treaty’s terms.

Russian officials have raced to portray the United States as the nation at fault.

They charge that American missile defense interceptors in Eastern Europe could be easily refashioned into offensive weapons, and that the rise of armed drones, which had not been invented when the treaty was signed, now threaten to provide Washington with similar intermediate-range ability without violating the precise wording of the treaty.

American senior national security officials, notably John R. Bolton, the national security adviser, have made no secret of their desire to leave arms control agreements that limit American action.

The question now is whether the United States will begin to deploy new weapons to counter China’s efforts to cement a dominant position in the Western Pacific and keep American aircraft carriers at bay. Much of Beijing’s growing arsenal currently consists of missiles that fall into the ranges — land-based missiles able to fly 300 to 3,400 miles — that are prohibited by the treaty.

China was still a small and unsophisticated military power when Ronald Reagan and Mikhail S. Gorbachev, the last leader of a rapidly weakening Soviet Union, negotiated the I.N.F. agreement.

The decision to leave it took American allies by surprise when word of the decision first leaked in October. Since then, European capitals have been caught between their fears of Mr. Trump’s unilateral instincts and their recognition that Mr. Putin poses a growing threat.

The North Atlantic Treaty Organization, the military alliance that was created to counter the Soviet threat 70 years ago, said on Friday that Russia was “fully responsible” for the breach of the treaty.

“The United States is taking this action in response to the significant risks to Euro-Atlantic security,” said Jen Stoltenberg, the secretary general of the Atlantic alliance.

Mr. Trump’s critics in Congress, however, immediately condemned the decision and pledged to introduce legislation seeking to stop the country from withdrawing from the treaty in six months.

Senator Edward J. Markey, Democrat of Massachusetts, who focuses on nuclear issues on the Foreign Relations Committee, called the treaty’s suspension “a tragedy that makes the world less safe.”

“President Trump and his war cabinet have yet again decided that America should go it alone, this time, by paving the way for a dangerous arms race,” he said.

Nuclear Weapons Inventories Around the World

Deployed

Other nuclear warheads in stockpile

Retired

Russia

6,850

1,600

2,750

2,500

United States

6,450

1,750

2,050

2,650

France

300

China

280

United Kingdom

215

Pakistan

150

India

140

Israel

80

North Korea

20

Deployed

Others in stockpile

Retired

Russia

6,850

United States

6,450

France

300

China

280

United Kingdom

215

Pakistan

150

India

140

Israel

80

North Korea

20

Note: Figures are estimates as of November 2018.

Source: Hans M. Kristensen and Robert S. Norris, Federation of American Scientists

By The New York Times

It seems unlikely that Mr. Pompeo’s announcement will result in a flurry of last-minute negotiations with Moscow, and it could accelerate the Cold War-like behavior among the United States, Russia and China. American intelligence agencies’ warned this week that Russia and China were “more aligned than at any point since the mid-1950s.”

The fate of the treaty has quickly become a test of the struggle inside the Trump administration, and with its allies, over how to handle an increasingly aggressive Russia.

Mr. Trump campaigned on remaking Washington’s relationship with Moscow; the open investigation by the Justice Department’s special counsel is, at its core, about whether he and his campaign aides promised to relieve sanctions and other impediments to Russia in return for financial or electoral benefits from Mr. Putin’s government.

Russia was first to have second thoughts about the I.N.F. treaty. It complained to the Bush administration 12 years ago that newer nuclear powers, chiefly China, were not constrained by its terms. They proposed expanding it to include other major nuclear states.

That logic, said Graham Allison, a Harvard professor known for his studies of nuclear strategy dating to the Cuban missile crisis, “is correct.”

If China refused, so would India, which feels threatened by Beijing’s nuclear fleet. If India failed to sign on, Pakistan would reject the limits.

Complicating the diplomatic issue is another, less publicized announcement that recently came from the Energy Department: For the first time in 28 years, the United States began producing a new warhead for a strategic weapon — the long-range weapons devised to reach Russia, China or North Korea.

The announcement was not entirely surprising. In early 2018, the Trump administration indicated in a nuclear strategy review that it would develop a new warhead for an unspecified number of Trident missiles, which are carried aboard submarines. The first of those weapons — called a W76-2 — is now being assembled at a weapons plant near Amarillo, Tex.

In an emailed statement on Wednesday, the National Nuclear Security Administration, which runs the country’s atomic weapons complex, said it was “on track” to complete the initial order and deliver the new warheads to the Navy by the end of this fiscal year. The number it is producing has been kept a secret.

The exact power of the weapon is also secret, but it is widely estimated to be roughly half the Hiroshima blast — what the Trump administration calls a “low yield” weapon.

The Trump administration has argued that it must match Russian strides in low-yield weaponry to deter conflict, by helping “ensure that potential adversaries perceive no possible advantage in limited nuclear escalation.”

But critics see the new class of strategic weaponry as making unthinkable outcomes far more likely — not less. If an American submarine ever fires a Trident missile with a low-yield warhead, critics predict that could provoke a nuclear retaliation before impact.

The cost of the W76-2 and the administration’s other new weapons are raising the price of making over the nation’s nuclear arsenal.

Last week, the budget office updated its projections to account for the coming decade though 2028, lifting its estimate by $94 billion.

The House, now under control of the Democrats, is vowing to review that budget. The buildup could result in a new global arms race; it might also be used as a bargaining chip to produce an arms control breakthrough.

Last year, when Defense Secretary Jim Mattis testified on Capitol Hill about the administration’s new arms policy, he alluded to a benign outcome. “We have to give our negotiators,” he said, “something with which to negotiate.”

A version of this article appears in print on , on Page A1 of the New York edition with the headline: U.S. Plans to End Nuclear Accord Signed by Russia. Order Reprints | Today’s Paper | Subscribe